1. Field of the Invention
The present invention relates to a method of making a stator vane array for the compressor or turbine of a turbomachine, as well as to the vane units and arrays or array sectors which are thus produced.
2. Summary of the Prior Art
The flow straightener stator stages of high pressure compressors and of turbines are at present made by one or other of two methods.
In the first method the flow straighteners consist of vanes which are cut to shape and assembled in sectors by brazing in punched rings. The machining of the blades is carried out on extruded bars from which a root is cut. The rings themselves are formed by rolling, and then punched. The assembly is effected by brazing the roots in the cut sections of the inner ring and brazing the upper ends of the blades in the cut sections of the outer ring.
This method has a degree of simplicity but its major drawback is that it only permits the manufacture of flow straighteners having blades of simple shape with cylindrical profiles, i.e. of which all the generatrices are parallel to each other.
However, the aerodynamic stresses suffered by the flow straighteners of compressors in high compression rate and high output turbomachines have resulted in seeking to make the flow straightener blades with an evolutive profile and chord. Thus, the blades may have curved ends for transition with the inner and outer rings.
It is not feasible to make such flow straighteners by the above-described method. Thus, a second method of construction is used which consists of casting, by the lost wax process, a complete array, or sectors which will be assembled together to form an array. This method, although enabling complex parts to be made, has all the drawbacks of precision casting. Accordingly it requires making a model of the array or of the sectors by assembling models of the individual blades with the inner and outer rings or ring sectors. From this model a shell mould is made by building up successive layers of ceramic on the model, destroying the model by melting, and then firing the shell mould. The molten alloy from which the array is to be made is then poured into the shell mould, and after solidification of the alloy the mould must be destroyed in order to recover the cast blank of the array or sector. The blank is then subjected to machining of the circular bearing surfaces, and, in the case of sectors, a machining of the side faces, before arriving at the finished array or sector.
This method involves considerable manufacturing stages (typically taking more than 8 months). Also, it makes use of costly industrial means (vacuum melting furnaces, furnaces for firing the shell moulds), the investment in which is justified only for substantial volumes of production.
The object of the present invention therefore is to provide an alternative method for the manufacture of flow straightener stages for high pressure compressors, or turbines, which requires the use of only "light" industrial means compared with those used in casting and also makes it possible to reduce the length of the manufacturing cycle, while enabling the blades to be made with an evolutive profile and chord.